Hollow glass microspheres (e.g. 0.5 mm) are used, for example, as bulk, lightweight fillers for reinforced plastic components for the motor industry. They find use also as hollow microbubbles for adhesives. They can be prepared, for example, by pulsing an inert gas, such as argon into a continuous stream of molten glass in order to produce individual microspheres. For best results, the molten glass should be at a temperature such that the melt has a viscosity of about 2 Pas (20 poise). This temperature is known as the 20 poise temperature (20 PT). The glass compositions previously proposed for use in the preparation of hollow glass microspheres in this way have had 20 PT values in the region of 1440.degree. to 1450.degree. C. For example, a standard E-Glass composition (analyzing 53.4 wt % SiO.sub.2, 13.9 wt % Al.sub.2 O.sub.3, 8.9 wt % B.sub.2 O.sub.3, 21.7 wt % CaO, 0.3 wt % Na.sub.2 O and 0.7 wt % K.sub.2 O), such as is used in providing glass fibers for reinforcing plastics, has a 20 PT of 1447.degree. C. A borosilicate glass composition used in the preparation of glass microspheres (analyzing 44.7 wt % SiO.sub.2, 12.5 wt % Al.sub.2 O.sub.3, 14.1 wt % B.sub.2 O.sub.3, 26.3 wt % CaO, 0.6 wt % Na.sub.2 O and 0.7 wt % K.sub.2 O) has a PT of 1444.degree. C. Such high 20 PT values can cause a reduced service life for metal parts, such as platinum bushings, employed in the manufacture of microspheres and are of course associated with high energy requirements. Accordingly it is desired to provide glass compositions suitable for use in the preparation of glass microspheres but having lower 20 PT values than the glass compositions hitherto used.